Generally, expandable particles of a composite resin of a polyolefin-based resin and a polystyrene-based resin (also referred to simply as expandable composite resin particles) is obtained by impregnating composite resin particles (also referred to simply as resin particles) with a volatile blowing agent. In addition, it is known that a moisture is included in expandable composite resin particles for improving moldability of an expanded molded article and the like (see, for example, Japanese Examined Patent Publication No. Hei 6-104746: Patent Document 1 and Japanese Examined Patent Publication No. Hei 7-91405: Patent Document 2).
Moisture contents of the expandable composite resin particles described in Patent Documents 1 and 2 are 0.5 to 1.5% by weight, and the volatile blowing agent used in Examples is butane.